Antimicrobial potential of bacterial bioactive molecules against pathogens with relevance for aquaculture.

A mucus sample obtained from an Antarctic sponge and a total of 98 bacterial isolates of different origin (32, 66 mesophilic and psychrophilic isolates, respectively) were randomly selected to perform a screening for bioactive molecule production, with particular regard to the antibacterial activity against bacterial pathogens with relevance for aquaculture. By the cross-streak method, about the 30% of screened isolates showed antibacterial activity against at least one target, and were successively subjected to the overlay test. Mesophilic strains generally resulted active exclusively against the target Photobacterium damselae subsp. piscicida, whereas thermophilic and psychrophilic strains showed a broader antibacterial activity. In a second step bacterial supernatants (pure and concentrated), obtained after incubation at different growth conditions, were screened for antibacterial activity, and in the case of a positive response the biomolecules were extracted. Among bacteria of polar origin, the concentrated supernatants of Salinibacterium sp. C3W3 and C2W9 resulted active against Phobacterium damselae subsp. piscicida after growth in presence of sucrose (10±0.0 mm). The same target was inhibited by concentrated supernatant of the mesophilic Bacillus sp. PS62 after growth in presence of sucrose and glucose (12±0.0 mm). Bacillus sp. PS62 produced a maximum extract amount of 20 mg/ml. The sponge mucus provided an extract that was active against V. anguillarum, A. salmonicida, and P. damselae piscicida (10, 15 and 20 mm, respectively). Our results suggest that bacteria could represent an optimal source of compounds active against bacterial pathogens relevant in the aquaculture field.